Effect of Focal Spot on Resolution (Magnification Radiography)

The radiograph shown above was obtained in magnification mode, where the distance from the focal spot to the image receptor was 94 cm, and the image from the focal spot to the foot phantom was 70 cm. The image magnification is thus 94/70 or 1.34. The radiographic technique used was the same as shown in the contact radiography (see above) or 50 kV and 7 mAs. The small focal spot was used to generate this image, and inspection of the line pair phantom shows that the limiting spatial resolution is ~ 3 lp/mm, or slightly less than achieved in contact radiography.

This magnification radiograph is identical to the one shown above, except that the large (1.2 mm) focal spot was used. The limiting spatial resolution has now fallen to 1.6 lp/mm, and showing the marked degradation due to focal spot blur. Measurement of the limiting spatial resolution in the line pair phantom image needs to be performed with great care, and aliasing artifacts need to be taken into account. In viewing the line pair phantom image, the pattern of black and white lines disappears at a spatial frequency of ~1.6 lp/mm, which is the true limiting resolution. The line pair phantom subsequently reappears and seems to show lines up to about 3 lp/mm; however, this pattern is a mirage which occurs due to aliasing, and does not represent the true limiting spatial resolution.